I have 11 sets of radio buttons corresponding to 11 questions on my survey form. I'd like to have my Error page be triggered by the case of someone filling out the survey but leaving a radio button set unselected. Can I do this?

There is no validation for a radio buttons because it always has one selection, so the user cannot do anything wrong. This is standard browser/HTML behavior.
If you want to implement different behavior then you will have to create a validation script yourself.
Usually this is done via the 'onsubmit' event of the form (for advanced users only...)